1. Field of the Invention
The present invention relates in general to a novel process and apparatus for drying sheet materials, such as paper and some types of wet process-nonwoven fabric, which are susceptible to breakage in the wet conditions as opposed to woven fabric, and which are subjected to intra-fiber contraction due to the formation of inter-fiber hydro-bonding caused by vaporization of moisture during drying process.
2. Conventional Art
A typical method for drying a paper web on a paper-making machine generally involves steam-heated dryer cylinders. In recent years, a closed dryer hood insulated by suitable insulating materials is installed so as to surround dryer cylinders in series. A large volume of high temperature, low humidity air (heated with exhaust steam and fresh stream) is blown into the dryer hood, and the moisture released from the paper web is removed in a moving air stream kept at no greater than a 70-80% saturation curve to prevent condensation inside the hood. However, the technique is energy-intensive because of the blowers needed to handle a large amount of process air, and is wasteful of energy also because the moisture-laden air is largely exhausted to the atmosphere except for a minor use for reheating of the fresh air supply.
Another technique developed in recent years involves the use of two heated and cooled endless steel belts holding a wet paper web therebetween. The moisture vaporized by the heated belt is immediately condensed on the cooled belt, and the condensate is drained onto an endless fabric belt. Thus, the wet paper web is dried at a temperature in excess of 100.degree. C. while restraining the expansion/contraction thereof. The paper dried by this technique is of high quality, but the technique requires a large quantity of cooling water, which can only be recycled as warm water. Therefore, the economy of the process is quite inferior.
For drying of thin papers, such as toilet and tissue papers, a wet web of paper is dried on a single large-diameter dryer cylinder, called a Yankee dryer, which has a canopy hood disposed so as to surround a top half portion of the cylinder. Fresh air and recycled moisture-laden air are heated to a high-temperature gas at about 300.degree.-450.degree. C., and are blown towards the exposed side of the wet paper web at a high speed of 70-120 m/s. The product has a Yankee glazed surface on one side only, and the other side remains rough. The use of manufactured product is therefore limited somewhat to such uses as a wrapping paper with one glazed side and a tissue paper.
Yet another drying technique proposed is based on not supplying air to the closed hood but recycling the saturated steam produced by vaporized moisture as a part of the heat supplied to drier cylinders, which are pressurized vessels. However, in practice, it is difficult to eliminate air completely from the closed hood, and furthermore, volumes of air enter into the closed hood by the continuous feed of wet paper web and endless fabric belt. Another problem of air entry into the hood occurs when the paper inside the hood breaks due to shrinkage, and it is necessary to open the closed hood. After the interior of the hood has been cleared of the breakage, the hood is closed and is re-started. Under these circumstances, it is impossible to keep air entering into the hood below 4% as is generally recommended. Further, the saturated steam inside the hood becomes condensed when cooled by the wet paper load as well as external air entering the hood. Then, condensation occurs on the metal surfaces of the hood and dryer frames, and the condensates may drip onto the dried paper creating staining defects and low yield. Because of such inherent problems, this proposed technique has not been commercialized yet.
In view of such problems in the existing techniques of paper drying, the present inventor made a detailed study of the current process of paper drying, and a summary review of the current problems is presented in the following.
The current effort to dry a wet paper web on a production scale is generally based on causing the wet paper web to pass on dryer cylinders in series, while letting both sides of the wet paper web alternately come into contact with the dryer cylinders so as to produce smooth surfaces on both sides of the paper to avoid curling or cockling. Furthermore, dimensional stability is provided by interposing the wet paper web between the cylinder and the endless fabric belt so as to restrict the free shrinkage of the wet paper web. However, inasmuch as most of the drying action is performed during a free running zone between the adjacent dryer cylinders, where the paper web shrinkage is not restrained, such attempt is not sufficient.
Furthermore, with increasing production speed of the paper-making machine, the number of dryer cylinders has also been increased nearly from several tens to one hundred cylinders. However, with an increasing number of dryer cylinders, operational and maintenance problems have also increased. For example, sectional drive system is implemented to impart uniform tension to the paper web to cope with the shrinkage in the passing direction. Furthermore, suction canvass rolls, air boxes as well as endless fabric belts are employed to prevent paper breakage and achieve evenness of drying in both longitudinal and transverse directions. Nevertheless, breakage of paper does occur frequently between the dryer cylinders or dryer sections, and when the paper web is broken, the paper-making machine must be stopped, and the closed hood must be opened to remove the breakage before the machine can be re-started. The existing process therefore demands much attention and manpower, and the maintenance problems can present problems of personal safety in some cases.
The increased size of the dryer cylinders presents performance problems also. The dryer cylinders have reached a diameter size of 1.2 to 1.5 m and even 2 m, and the cylinder width has also been increased to a size in excess of 10 m. The steam pressure in the dryer cylinders, which are formed of castings, has reached 2-4 kg/m.sup.2 G. With increasing productivity demanded of the dryer cylinders, problems have emerged that it is difficult to collect the condensate inside the dryer cylinder because the condensate rotates with the interior surface of the dryer cylinder (rimming condition) due to centrifugal force by the increased rotational speeds. With high speeds of operation, draining of the condensates does not take place smoothly, which presents a problem of uneven condensate layer resulting in uneven moisture across the paper width.
Another serious problem is associated with the consumption of energy needed to produce a huge volume of steam required for the drying operation. Depending on the product, 1.5-3 tons of steam is required for every ton of dried paper produced. The performance of the hood has been improved in recent years by improving the insulation of the hood so as to obtain a dew point of around 65.degree. C., and the volume of air required has also been lowered significantly. However, most of the steam vapor evaporated is still exhausted to the atmosphere, and a problem remains of generation of white smoke produced by condensation of moisture in the exhausted moist air, particularly during winter and early spring seasons. In some locations, this presents a serious hazard to residents and traffic.
A further important problem associated with the conventional drying process is the dew point of the carrier gaseous stream. So long as moist air is used as the carrier stream for the vaporized steam, the upper limit of dew point is around 65.degree.-70.degree. C. When the volume of dry air is low relative to the volume of the vaporized steam to be carried, saturation of the carrier air can occur easily, and condensates are produced inside the closed hood. The condensates dripping on the dried paper will produce rejects, and poor yield will be the result.
Another processing problem related to the method of drying is inherent in the conventional drying system. Specifically, the temperature of the side of the wet paper web in contact with the dryer cylinder reaches about near 100.degree. C., but the side contacting the fabric belt can only reach a temperature of about 90.degree. C., because the fabric belt is wetted with the moisture removed from the paper evaporated by the cylinder. Furthermore, because the fabric interior is wetted with moisture, there is a high temperature gradient between the outside layer of the fabric in equilibrium with moist air (65.degree.-70.degree. C.) and the inside layer in contact with the paper web (about 85.degree. C.), thus greatly impeding rapid evaporation of water from the paper web. For this reason, there is little drying taking place in the zone of the dryer where the wet paper web is in contact with the fabric, and most of the drying actually takes place in the free running zone between the dryer cylinders, where the moisture is evaporated directly from the heated paper web. It is, indeed, estimated that about 80% of the moisture is evaporated in the free running zone and only about 20% of drying takes place in the fabric-restrained zone of the dryer. Therefore, the effort to improve the dimension stability of the paper to prevent shrinkage by providing the fabric-restrained zone appears to be largely wasted.